The present invention relates to Mach-Zehnder interformetric devices and to methods of making the same.
Optical fiber communication systems require wavelength selection devices for various purposes. For example, in a wavelength division multiplexing transmission scheme, a single fiber may carry several beams of light at slightly different wavelengths. Each beam carries a separate stream of information. A wavelength selective filter is used at a point where the fiber branches to direct one beam at a particular wavelength onto one branch of the fiber and to direct the other beams onto the other branch. Several receivers belonging to different telecommunications customers can be connected to a single fiber. Each receiver is equipped with a filter adopted to direct only a very narrow band of wavelengths to that device and to exclude all others. Signals intended for the particular subscriber are sent at the wavelength associated with that subscriber.
These and other wavelength selective devices must meet demanding requirements for use in practical communications systems. The devices should be capable of separating wavelengths differing from one another by only a few nanometers. The wavelength selective device should be environmentally stable, reliable and durable. In some applications, the wavelength selective device should be "tunable" or adjustable to vary the wavelengths which it selects. Also, the wavelength selective device should operate with a relatively low loss of optical power, i.e., the device should not dissipate substantial amounts of the optical power supplied to it.
Mach-Zehnder interferometers have been utilized as wavelength selective devices in optical communication systems. As depicted in FIG. 1, a Mach-Zehnder interferometer includes a pair of fibers F1 and F2. The fibers are coupled to one another for light transfer therebetween at a first coupler C.sub.1 and a second couple C.sub.2. The couplers are arranged to transfer light, one fiber to the other. As further explained below, the couplers may be so-called "evanescent" couplers in which narrowed, elongated portions of the fibers are closely juxtaposed with one another within a matrix or outer coating. The couplers may be 3dB couplers, arranged to transfer approximately one-half of the optical power supplied on one fiber to the other fiber. Fibers F1 and F2 have phase shift regions with different optical path lengths disposed between the couplers. Thus, the optical path length over the phase shift region in fiber F1 is different from the optical path length over the phase shift region in fiber F2. As used in this disclosure, the term "optical path length" is a measure of the time required for light at a given wavelength and in a given propagation mode to pass through the fiber from one end to the other. The optical path length difference has been provided either by making the phase shift region of one fiber physically longer than the other, or by making the two fibers F1 and F2 with different propagation constants so that the velocity of light within the two fibers is different. The latter structure can be effected by making the fibers with different refractive index profiles. Where the fibers are "step-index" fibers, incorporating a core having a relatively high refractive index and a coating with a relatively low refractive index overlying the core, the two fibers may have cores of different refractive indices, different core diameters, different coating refractive indices or some combination of these. Regardless of the particular mechanism used to produce the optical path length difference, the single stage Mach-Zehnder filter depicted in FIG. 1 will direct light supplied through input 1 either to output 3 or to output 4 depending upon the wavelength of the light. More complex Mach-Zehnder devices utilize multiple stages with multiple phase shift regions and multiple couplers connected in series to achieve certain desirable wavelength-selective characteristics. Still other Mach-Zehnder devices incorporate more than two fibers connected in parallel between the couplers, as described in U.S. Pat. No. 5,351,325, the disclosure of which is hereby incorporated by reference herein. The various optical fibers incorporate different optical path lengths. Desirably, the optical path length differences are selected to provide optical path length differences which are rational or integral multiples of one another.
For Mach-Zehnder devices to provide the desired wavelength-selective characteristics, the path length differences should be as specified in the design device and should remain stable except when deliberately altered. Environmental effects, such as movement or vibration of the individual fibers, and differential heating or cooling of the fibers can severely degrade the performance of Mach-Zehnder components. U.S. Pat. No. 5,295,205 (`the '205 patent"), the disclosure of which is also hereby incorporated by reference herein, discloses an improved Mach-Zehnder device incorporating an elongated body of a matrix glass formed as a hollow tube. The optical fibers extend through the bore of the tube. Each coupler may be formed by collapsing a portion of the tube onto the fibers, as by heating it, and then stretching a portion of the collapsed tube, and portions of the fibers contained therein, to provide narrowed, elongated sections in the fibers surrounded by the matrix glass. This general approach can be utilized to form a wide variety of Mach-Zehnder components, including those having more than two fibers and staged devices having more than two couplers. The devices formed in accordance with preferred embodiments of the '205 patent are securely encased within the matrix glass tube and hence are substantially insensitive to temperature gradients and undesired, inadvertent bending. The preferred devices formed according to the '205 patent, therefore, can be used as components in practical telecommunication systems.
Despite these and other advances in the art, there is a need for further improvement. Manufacture of Mach-Zehnder devices using fibers with different propagation constants requires the manufacturer to stock fibers having different propagation constants. When more than two fibers are employed in a single device, the fibers must be made with propagation constants having the desired relationship to one another. For some designs, the fibers must be made in sets with differences between propagation constants such that the differences are integral multiples of one another. This imposes significant constraints and costs in the fiber drawing process. Moreover, the adjustments to the fibers required to achieve the desired propagation constants can have undesirable side effects. For example, adjustment of the fiber core composition to yield a particular propagation constant can yield a fiber having a particularly soft core which forms to an elliptical cross-section during the stretching process used to form the couplers. This, in turn, can result in optical performance which varies with the polarization of the light transmitted through the device. Accordingly, further improvements in methods of making Mach-Zehnder devices and in the resulting devices would be desirable.